The invention concerns a magnetic resonance (MR) probe head with a detecting device, comprising at least one antenna system which is cryogenically cooled by a cooling device, and a cooled preamplifier in a preamplifier housing which is disposed at a spatial separation from the detecting device, and with a thermally insulating means which connects the detecting device to the preamplifier housing, wherein the connecting means comprise at least one cooling line for supplying and/or returning a cooling fluid, and at least one radio frequency (RF) line for transmitting the electrical signals.
A probe head of this type is disclosed e.g. in reference [3] (see e.g. FIG. 5 thereof) and is used for detecting NMR signals from a sample. The receiver coil is thereby cooled. This receiver coil may e.g. be a coil made from a normally conducting metal, e.g. copper or aluminium, or from superconducting materials, in particular, high-temperature superconductors (HTSC).
The conventional cooled systems for NMR detection can be classified into the following categories:    a) systems which are directly cooled by LN2. LN2 is thereby inserted into a container and measurements are performed until the coolant is consumed (evaporated). Such an arrangement is mentioned e.g. in reference [1] and is schematically shown in FIG. 5.    b) systems which are cooled by a cold head in the direct vicinity of the receiver coil, generally using a so-called pulse tube cold head, which is compatible with the requirements for the magnetic field and vibration. A configuration of this type is disclosed in reference [2] and schematically shown in FIG. 6.    c) systems which are cooled by a cold head, wherein the cold head is located at a certain distance from the probe head itself. This cold head is typically a Gifford-McMahon cooler, but may also be a pulse tube cooler. The probe head is generally cooled with a flexible transfer line to bridge the separation from the cryocooler and to also keep vibrations away from the probe head. A heat transporting medium circulates in this transfer line (usually cold, gaseous helium). The probe head itself typically contains the receiver coils and the preamplifier, which are connected to each other via RF lines. The probe head is thereby designed as a rigid unit. Such a configuration is disclosed in reference [3] and represented in FIG. 7b (horizontal magnet) or in FIG. 7a (vertical magnet).
It must be noted that this concerns only reception of the NMR signals. This is also the process which must be controlled with maximum precision to obtain a maximum S/N ratio, and depends to a critical degree on attenuation, temperatures and noise of the elements involved.
The nuclei must be excited prior to measurement by at least one RF pulse. This can be realized either by the receiver coil itself or through a separate transmitter coil. The transmitter coil may either also be cooled or be at room temperature. Further systems and devices for generating the nuclear excitation are not discussed herein for reasons of simplicity.
The above-mentioned conventional MR probe heads are discussed below:    a) the configurations in accordance with a) (FIG. 5) have a relatively simple construction but are disadvantageous in that liquid nitrogen 60 must be refilled into a thermally insulated housing 61 and the temperature of the receiver coil 5 is only reduced to 77 K. It is thereby important that the preamplifier 58 and also the RF line 59 (usually in the form of a coaxial cable) between the receiver coil 5 and the preamplifier 58 are also normally at room temperature. In consequence thereof, the preamplifier 58 produces an unnecessary amount of noise and the RF line 59 additionally impairs the S/N ratio, for the following reasons: Firstly, the NMR signal itself is weakened by attenuation in the RF line 59. Secondly, the RF line 59 itself generates an undesired noise signal due to its dissipation. This is higher, the higher the RF line 59 attenuation and the higher its physical temperature. These disadvantageous effects are multiplicative. An RF line 59 at room temperature has the following disadvantages: its signal losses are high, and its physical temperature is also high, thereby providing two counts for of high noise emission. For this reason, the above-mentioned effects will be substantial.    b) The configuration of b) (FIG. 6) is a potentially better solution in view of cooling. The system may thereby be very compact, wherein cooling takes place in the vicinity of the receiver coil 5 to facilitate obtaining a low temperature. The system is also a closed system which requires only electric energy for driving the compressor 56 required for operation of a pulse tube cold head 54 in a thermally insulated housing 55 and is connected to the pulse tube cold head 54 via a flexible pressure line 57. In particular, no cryogenic liquids must be refilled.
The configuration has one serious drawback: the preamplifier 58 must either be operated at room temperature and/or outside of the magnet 1. In the first case, its noise temperature is not optimum, but excessively high. In the latter case, the preamplifier 58 could also be cooled. There are no conventional preamplifiers with satisfactory function which perform perfectly at cold temperatures (<<77K) in the magnetic fields (>>1T) used for NMR, in particular, high field NMR.
When the preamplifier 58 is outside of the magnet 1, it is conventionally connected to a receiver coil 5 via a correspondingly long RF line 59. This RF line 59 is at room temperature, and therefore has the disadvantages mentioned under b) above, leading to deterioration of the S/N ratio.
There is another problem. In order to cool the preamplifier 58, it must either be cooled by LN2, which is not very user-friendly and opposes the original idea of a closed system, or a second pulse tube cold head is used only for the preamplifier 58. This is very expensive. In any case, the problem of the non-cooled RF line 59 remains.    c) the configuration in accordance with c) (FIG. 7a, FIG. 7b) is technically the best of the conventional configurations and potentially offers the highest performance, since the RF receiver coil 5 is operated in the room temperature bore 2 of the magnet 1 at a very low temperature (e.g. 20K) and the preamplifier 16 is operated at a such a low temperature to assume proper function (e.g. 77K). Also the RF line 52 between the receiver coil 5 and the preamplifier 16 is at a very low temperature. The temperature typically changes along the RF line 52 from 20 K to 77 K, wherein the geometry (in particular the cross-section of the cables of the RF line 52) is selected to obtain an optimum between minimizing the RF line losses and minimizing the heat input into the cold receiver coil 5 produced by the heat conductivity of the RF line 52. Thus, the S/N ratio of the system is nearly optimum. After amplification with the cooled preamplifier 16, the signal is finally output to a signal output 17 at the output of the preamplifier 16 for further signal processing. The overall cooling can be performed with one single cryocooler 20 (Gifford-McMahon or pulse tube cold head).
This configuration has been prior art for some time for high-resolution probe heads of NMR spectroscopy (FIG. 7a) [3]. Prototypes for MRI imaging have also been built according to this principle (FIG. 7b) [4]. This configuration has the serious drawback that the entire probe head is long, which produces the following problems:    1. Installation may become impossible under unfavorable spatial conditions. These problems are shown in FIG. 8. The separation D1 between the magnet 1 and the wall 62 must be sufficient to permit installation and removal of the system into and from the magnet. This minimum distance is substantially determined by the sum of the lengths of the actual detecting device 3, the preamplifier housing 15a, and the rigid connecting means 15c.     2. The entire probe head including typically large preamplifier 16 and associated housing 15a is quite heavy, such that installation/removal typically requires two persons.    3. Due to the large weight and bulkiness of probe head and connected cooling line, installation or removal of such a probe head in the cold state is very difficult and not practicable. The transfer line 19 (FIGS. 7a, 7b) which extends between the cryocooler 20 and the probe head, typically contains four cooled pressure lines (continuing cooling lines 21a-21d) and is quite heavy and inflexible. Installation/removal in the cold state is desired and would permit cooling the probe head outside of the room temperature bore of the magnet prior to use, and insertion of it into the room temperature bore only directly before the measurement. After the measurement, the probe head could be removed in the cold state and be left cold until subsequent use or allowed to heat up outside. This is not practicable for the above-mentioned reasons, and for this reason, the probe head must be typically inserted into the room temperature bore 2 in a warm state (i.e. at room temperature), and then connected to the transfer line 19 and the cryocooler 20. Only then can the cooling process start, which typically takes at least four hours. During this time, the magnet 1 is blocked and unproductive. Removal is effected under similar conditions. The warm-up phase typically takes at least two hours. These times cannot be tolerated in view of the required utilization rate of an MRI system.
Prior art provides either relatively compact, cooled systems without optimum performance or high-performance systems with optimum little noise. These may, however, be fundamentally unsuitable for installation into narrow spaces, or be very difficult to handle.
It is therefore the underlying purpose of the present invention to propose an MR probe head which is easy to handle, is highly sensitive, and permits rapid installation, removal and rapid start of operation of the detecting device.